Particulate Matter (PM), particularly fine/ultrafine PM, has been associated with an increasing number of adverse short- and long-term health effects, particularly morbidity and mortality from cancer and cardiopulmonary diseases [1-3]. It has become evident that systemic inflammation and oxidative stress play key roles in the pathogenesis of these diseases and may serve as the common mechanisms by which PM potentiates these diseases. However, in vitro and in vivo studies can only provide indirect evidence due to the inherent uncertainty in the approaches when extrapolating their results to humans. An air quality improvement initiative in Beijing during the 2008 Olympics created a unique natural experiment with an initial dramatic decline in air pollution concentrations followed by a return to pre- Olympic concentrations. We took advantage of this unique opportunity, and designed a prospective cohort study in Beijing to investigate the acute biological response to changes in human exposure to PM and to better understand the critical pathways through which PM operates in these diseases. The specific aims of the proposed study are to examine whether changes in PM exposure over the course of the Olympics are related to changes in selected markers of DNA/lipid/protein damage and antioxidant defense, or to changes in respiratory and systemic inflammatory response. To achieve the proposed aims, we enrolled 201 adult males and females prior to the air quality improvement initiative in Beijing, China and followed these individuals over the course of the Olympics. One hundred eighty participants completed a series of three interviews: before, during and after the Olympics. Each interview consisted of an in-person interview, physical examination, and biospecimen collection (blood, urine and sputum). Ambient PM concentration in the study area was measured throughout the study period. Multianalyte multiplexed assays are proposed to analyze the selected inflammatory markers. Automated enzyme kinetic analysis, HPLC, ELISA and EIA will be used to assess oxidative DNA/lipid/protein damage and antioxidant defense. We predict that we will observe a decrease in the levels of markers for systemic inflammation and oxidative damage in response to improvements in air quality, and an increase in the levels of anti- inflammatory cytokines and antioxidant enzymes in the first follow-up period. Our hypotheses will be further evaluated by examining changes in inflammation and oxidative damage as air quality in Beijing returns to pre-Olympic levels in the second follow-up period.